The research is intended to delimit the relative contributions of the "descending" and "supraspinal" components of the endogenous neuronal mechanisms which contribute to stimulation-produced analgesia (SPA) when activated by electrical stimulation applied in midline brainstem structures implicated in this phenomena (The periaqueductal gray matter, the raphe nuclei and the locus coeruleus). The study involves combined behavioral and physiological studies performed in the same animal. We employ a paradigm whereby the animal (cat) is allowed to obtain a reward (milk) in exchange for tolerating a noxious stimulus (foot shock). Simultaneously, compound action potentials indicative of the responses of higher order sensory neurons of the dorsal horn are recorded from the spinal cord. Electrical stimulation is applied through electrodes implanted in brainstem structures implicated in SPA. The effects of the brainstem stimulation on the escape threshold from the foot shock is compared with its effects on the potentials recorded from the cord. In this way we can infer the relative contribution of descending and supraspinal mechanisms. Sites in the brainstem from which strong SPA can be evoked will also be tested after administration of drugs which block neurotransmitters implicated in key roles in SPA. (Serotonin, the endogenous opiate ligands, and noradrenalin). Thus we may isolate pharmacologically components of the descending and supraspinal mechanisms contributing to the SPA evoked from that particular site.